Heat Illnesses

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Author: Ian Rogers, MMBS FACEM, Professor of Emergency Medicine, St John of God Murdoch Hospital & University of Notre Dame Fremantle

Heat related illnesses occur in many forms. Most of these conditions can occur in the absence of heat stress, with the heat simply serving to exacerbate them. Examples include cramps, dehydration and impaired heart function. The one true heat related emergency is heat stroke, a life-threatening condition that requires urgent recognition and treatment in the field. In general the risks of heat illness are greatest with intense exercise in hot, humid environments with high levels of UV radiation. 

Prevention of heat illness

To prevent or minimise the risk of heat illnesses consider the following issues:

  • Dress in light coloured, loose garments to facilitate sweat evaporation, which is the main means used by the body to dissipate heat. Excess body fat provides insulation which reduces heat loss. The face, scalp and upper torso are the most important areas to provide cooling through sweating, so prudent removal of headgear and towelling off improves heat loss.
  • Modify the level of physical activity according to the heat and humidity, taking frequent rests in the shade, and consider the best time of the day to exercise and to rest.
  • Physical fitness results in improved exercise tolerance in the heat, but even fit individuals may lack conditioning for a particularly strenuous activity or adverse environmental conditions. Paradoxically the fit may be more at risk because greater ability to exert and increased muscle bulk means more heat can be generated. 
  • Acclimatisation, improving the body’s response to heat, occurs with gradual increase in time and intensity of exercise in the heat, usually requiring about 1 hour of exercise per day over a period of 8–10 days. Once acclimatised, the body sweats sooner and in greater volumes, facilitating body cooling. Loss of acclimatisation to heat takes about 1–2 weeks. Acclimatisation is not improved by restricting fluid intake.
  • Fluid needs to be available to ensure the group can drink freely to satisfy their thirst. This ‘drink to thirst’ strategy is the best way to avoid the risks of both under and over hydration (see below). 

Medical conditions may make someone more susceptible to heat illness, either because of a limited ability to sweat  (eg antidepressant drug use, old age) or excessive heat production/retention (eg amphetamine use, obesity). Generally the very young and the elderly are considered most at risk of heat illness. 

Fluid intake and exercise

A ‘drink to thirst’ strategy is the best approach to fluid consumption during exercise. 

Our understanding of appropriate fluid balance for exercise has changed fundamentally. 

Over recent decades much was made of the need to aggressively consume fluids to maintain normal hydration and athletic performance. This was despite little evidence that mild dehydration (loss of up to 3% of bodyweight) had any significant effect on athletic performance or the risk of heat illness. 

It is now certain that the risks of over-hydration are at least as great as those of under hydration and that drinking to a predetermined rule is not the best way to manage fluid balance. Overhydration dilutes the sodium level in the body leading to brain swelling – so called exercise associated hyponatraemia (EAH). This is a life threatening condition and has led to the demise of many endurance athletes. Using electrolyte drinks does not prevent this but a ‘drink to thirst strategy’ does. 

Not surprisingly the human body is remarkably well adapted to sense and maintain hydration status. The best current advice is to ensure fluids are freely available and simply give the instruction to ‘drink to thirst’. What fluids are consumed is not important and no set rate can be given as every person and every situation is different. 

Sports drinks containing electrolytes confer no particular advantage other than that the carbohydrate they contain (usually at about a 6% concentration) may be a useful energy source and is the least likely to cause gastrointestinal distress.


Cramps are brief muscle spasms that may cause severe pain and usually involve heavily exercised muscles, with legs and abdomen most commonly affected.

Cramps usually follow several hours of sustained exercise of involved muscle groups, and are not solely due to heat exposure but are clearly more common in a hot environment. Cramps in the abdominal muscles may be hard to differentiate from gastrointestinal upset. Although this is a distressing condition, it is not life threatening.

Management and prevention of cramps

Cramps resolve with rest in a cool environment and with oral fluids. Whilst there is a widespread belief that salt supplements or solutions containing electrolytes such as sodium and magnesium can both prevent and treat cramps there is no consistent evidence for this. Using such approaches is unlikely to do any harm but cannot be recommended as evidence based. Gentle stretching or massage of the involved muscles often helps. Once recovery occurs the patient may resume activity, but if cramps return then they should rest for longer.

Exercise Associated Collapse

Exercise Associated Collapse (EAC) was previously called heat exhaustion. 

Exercise associated collapse occurs when the effect of muscle pumping returning blood to the heart is lost as exercise stops. The casualty may feel dizzy, slump to the ground or even lose consciousness.

The heart is a simple pump designed to push out on one side what blood flows into it on the other. During exercise the returning blood flow to the heart is enhanced by the pumping action of active muscles. When exercise stops the returning flow drops which leads to a drop in blood pressure. This blood pressure is essential to ensure adequate blood flow to the uppermost part of the body, in particular the brain. Unless the heart and blood vessels react quickly to correct this change in flow then brain blood flow will be inadequate and symptoms such as dizziness, nausea, collapse and ultimately loss of consciousness will ensue.

Many people will have some understanding of these sensations which are the same as that felt when stepping quickly out of a hot shower or rising from bed. The symptoms will be obvious to onlookers as the patient staggers, slumps to the ground and may become unresponsive. The symptoms should resolve rapidly once the patient collapses as the brain is now at the same level as the heart and the blood pressure will be sufficient to ensure adequate brain blood flow. This rapid recovery once lying down is the hallmark of EAC and helps to differentiate it from more serious illness. 

Whilst dehydration will make EAC worse it is not the primary cause and many patients with EAC will not be dehydrated at all. It is wrong to assume that because someone has suffered EAC that they must be dehydrated. Severe dehydration is needed before collapse occurs.

Management of Exercise Associated Collapse

Lying a patient flat should produce a prompt response.

  • Lie the patient down, ideally with the legs elevated. 
  • Within seconds, and certainly within a minute their conscious state should return to normal although it is quite likely they will not remember what has happened to them. 
  • Once consciousness is restored the treatment is rest and oral fluids until the patient feels well enough to get moving again.

The lack of a prompt response to lying down should lead to a consideration of more serious causes for collapse such as heat stroke, or low blood sugar in a diabetic. 

Heat stroke

Heat stroke is a true medical emergency with a high untreated mortality. 

In heat stroke, body systems lose their ability to regulate body temperature, and elevation of the core temperature follows, resulting in damage to many body systems, including the brain, and ultimately death from multiorgan failure. It is important to note that although maintaining hydration reduces the risk of heat illness, heat stroke is often not associated with significant dehydration.

The collapse of a previously healthy person associated with physical exertion in a hot/humid environment should be regarded as heat stroke unless there is another obvious cause.

The key findings in heat stroke are markedly elevated body temperature and altered conscious state

  • It is not important to try and measure the core temperature in the field as this is technically difficult without specialist resources. 
  • The skin looks red, typically feels hot, but may be dry or sweaty. 
  • The altered conscious state may range from irritability, confusion, hallucinations, poor coordination, aggression or bizarre behaviour to seizures and coma. 
  • There is a rapid pulse and breathing.

Management of heat stroke

Rapid cooling must be undertaken immediately and the most effective cooling strategy is cool water immersion. 

The chance of serious injury or death from heat stroke increases with the duration and height of elevated body temperature, so the more rapid the cooling, the lower the mortality rate. 

Cooling should include as many of following measures as possible:

  • Immediate rest
  • Shade from sunlight
  • Insulate from any hot surfaces (eg rocks) underneath the patient. Remove as much clothing as possible
  • Immerse the patient to the neck in cool water (improvise – this could be a dam, a stream, child’s wading pool or even a wheelie rubbish bin filled with water).

Should cool water immersion not be possible try these additional measures:

  • Sprinkle or sponge the patient with water
  • Fan aggressively using any improvised means (from a clipboard to a helicopter downwash!)
  • Periodic towelling off of the skin and rewetting improves evaporative cooling
  • Apply cold packs (ice if available!) to the neck, armpit and groin, where large blood vessels lie close to the skin, to facilitate cooling. If ice itself is available it may be best used to cool wet towels which can be placed on the patient for a few minutes at a time then recooled
  • Allow the air to circulate around the patient, while providing shade (e.g. tent flaps open).

Cooling measures should be tapered off once the patient feels cool and their conscious state begins to return to normal. Evaporative cooling is preferably with tepid water rather than cold, and this also reduces shivering, which is undesirable in heat illness as the muscle spasms produce heat. Watch for any return of high temperature, which would warrant renewed cooling.

Shock may develop in heat stroke due to heart failure and should be treated with standard patient stabilisation measures (danger, response, airway, breathing, circulation). Most patients with heat stroke are initially unconscious or semi-conscious and should not be given oral fluids.

Water is a key component to the management of heat stroke; lack of water greatly reduces the ability to cool the patient. Unfortunately, if trip planning and group preparation is poor, heat stroke may occur in circumstances where there is a lack of water. Further, exhausting water supplies to treat the patient with heat stroke places the remainder of the group at risk. 

The emphasis is on the need for planning and prevention. Urine may be used to moisten the patient and facilitate cooling. Do not give medications used to treat fever (such as aspirin or paracetamol) as these are not effective in heat stroke. 

Heat stroke is always an indication for urgent evacuation or rescue.